1. Field of the Invention
The present invention relates to a method of controlling the position of a mask in an exposure apparatus, and more particularly, to a method of controlling the position of a mask and a method for aligning the mask with a photosensitive substrate in an exposure apparatus having a mask stage whose position and angle in a two-dimensional plane are controlled by a mask interferometer.
2. Discussion of the Related Art
In a photolithography process in the manufacture of semiconductor devices, liquid crystal display devices, or the like, an exposure apparatus has been used for transferring a circuit pattern on a mask (reticle) onto a photosensitive substrate through a projection optical system. In particular, a step-and-repeat type reduction projection exposure apparatus (so-called "stepper") that drives the wafer stage serially or sequentially and operates in a batch exposing mode has been widely used.
In the reduction projection exposure apparatus of this type, when a circuit pattern on the reticle is projected on a wafer by exposure, the alignment between the wafer and the reticle in the rotational direction is performed in the following manner. Initially, searching alignment measurement is performed with respect to the wafer on a wafer holder, measuring the amount of rotation of the wafer from a reference position. On the basis of the measured result, the wafer holder is rotated so that the wafer is roughly aligned with the reticle. Then, fine alignment measurement is performed, and a reticle stage, which holds the reticle, is moved at the time of exposure so that the reticle is aligned with the wafer in the rotational direction with high accuracy.
In the conventional alignment method above, alignment in the rotational direction is implemented such that after the wafer holder is rotated, a remaining slight deviation in alignment between the wafer and the reticle in the rotating direction is removed by minutely rotating the reticle stage. One way to improve throughput is to combine the conventional two steps of operations for alignment in the rotational direction into one step, in particular, by unifying the two steps into operation of rotating only the reticle stage. Since a .theta.-stage, on which the wafer holder is mounted, is fixed onto an X stage by a vacuum chuck, the rotation of the wafer holder would require that this vacuum be turned off when rotating the .theta.-stage, and turned on again after the rotation. On the other hand, if only the reticle stage is rotated, such procedures are not required.
When the alignment in the rotating direction is performed by moving only the reticle stage, the amount of rotation of the reticle stage is considerably larger than that in the conventional method. It is difficult to rotate the reticle stage by a large amount without changing the conventional arrangement and alignment sequence, since such large rotation may degrade alignment accuracy for reasons as described below.
First, a reticle interferometer for measuring the position of the reticle typically measures a displacement of an apex in a corner mirror provided on the reticle stage in the direction parallel to the optical axis of a laser beam emitted from the reticle interferometer. Abbe error may occur if the apex of the corner mirror does not exist along the optical axis of the laser beam when the reticle stage is rotated.
Second, while the reticle is loaded by a reticle loader such that a reference edge of the reticle is positioned every time at the same place in the reticle stage, positioning errors may occur due to fluctuation of a pattern position on the reticle with respect to the reference edge of the reticle and poor reproducibility of the reticle position in loading the reticle. This results in positional variation of the reticle pattern with respect to the reticle stage. Accordingly, the position of the reticle stage need be adjusted to offset the variations so that the reticle pattern is brought into an optimum exposure position. As a result, the corner mirror of the interferometer (reticle interferometer) of the reticle stage inevitably deviates from the optical axis of the reticle interferometer. If a large amount of rotation is required in the reticle stage, the Abbe error may increase and the difference between the measured position of the reticle calculated through the reticle interferometer and the actual position of the reticle may increase, resulting in poor alignment accuracy.